Method and apparatus for cast-coating paper



Aug. 22, 1961 E. w. FREEMAN ET AL 2,997,406

METHOD AND APPARATUS FOR CAST-COATING PAPER Filed June 5, 1957 ATTORNEYUnited States Patent 2,997,406 METHOD AND APPARATUS FOR CAST- COATINGPAPER Eben W. Freeman, Portland, Lindsay 0. Golf, Westbrook, and RobertC. Rose, South Portland, Maine, assignors to S. D. Warren Company,Boston, Mass.,

a corporation of Massachusetts Filed June 3, 1957, Ser. No. 663,234 4Claims. (Cl. 117-64) This invention relates to a method of and apparatusfor treating paper to avoid stretching the paper web when it is passedbetween a rigid roll or drum and a yielding roll such as a rubber orrubber covered roll. When a paper web is passed between a rigid roll anda yielding roll under sufficient pressure between the rolls to causesubstantial deformation of the yielding roll there is a tendency tostretch the web. This is objectionable not only in that it tends toweaken the web and change its dimensions but the web may even be torn orcreped and especially in the case of coated paper strains may bedeveloped which cause the paper to warp or curl objectionably. Theharmful efl ects produced depend upon several factors such as whetherthe paper web is wet or dry, its strength and thickness, the amount ofdeformation at the roll nip etc.

The invention is particularly applicable in the finishing of cast-coatedpaper and will be fully described in connection therewith.

In the manufacture of drum-finished coated paper such as the papercommonly designated as cast-surface mineralcoated paper it is customaryto press against a moving, heated, highly polished metal surface, suchas the surface of a chromium plated drum or roll, a layer of wetmineral-coating composition supported by a Web of paper, the pressurebeing applied by means of a flexible rubbercovered roll acting on thereverse or uncoated side of the paper web. The term rubber as used inrubbercovered roll is intended to include both natural and syntheticrubber and similar el-astomers, usually reinforced with carbon black orother powder filler. The rubbercovered roll and the polished metal rollhave their axes parallel and horizontal so that when the rolls arebrought together they make contact in a rectangular area the long axisof which is a straight horizontal line. As pressure is applied betweenthe rolls the metal roll slightly sinks into the surface of therubber-covered roll to form therein an indentation or concave depressionso that the contact between the two rolls is not a mere line ofinsignificant width but is a bite or nip of considerable width, say from/s inch to 2 inches or more depending upon the magnitude of thepressure, the diameter of the rolls, and the hardness of therubber-covered roll.

It is apparent that since in a rotating roll all points on its surfacemust move with the same angular velocity it follows that as the radiusof the roll is shortened by compression the linear velocity at the pointof compression may be expected to be correspondingly decreased. On thisbasis, as a point on the soft roll surface enters a compression nip itwould be expected to show a slight decrease in speed graduallyto themiddle of the nip and then to show a corresponding slight increase inspeed during the remainder of the nip. In the past it has been believedby some people that the slight shortening of the radius of thepressure-roll has been the cause of the development of curling tendencythat frequently results from passage of paper through such pressure-nipsin certain coating operations, and particularly in cast-coatingoperations.

It has now been proved that the development of curling tendency bypassage through a pressure nip of the type described above is due not tothe change in radius of the pressure roll under compression, but to awholly diiferent and hitherto unrecognized cause. As a matter of factthe eifect of the radius change is exceedingly slight in most cases, butits action, such as it is, is to oppose or decrease the elfect of thenewly recognized major cause of curl development in the pressure nip.

Ourl is particularly troublesome in the case of drumfinished coatedpaper because after the wet coated paper has been pressed against themetal finishing surface it is allowed to remain in contact therewithuntil it has become substantially dry. No further finishing treatment iscustomarily given the drum-finished paper subsequent to the dryingthereof; so there is no opportunity to alleviate the curl after thedrying process as there is in the case of other coated papers which aresubjected to calendering operation after they have been dried. This isnot to intimate that exceedingly bad curl can be wholly removed orcontrolled by a calendering treatment, but it can at least be somewhatlessened thereby.

In the production of cast-surface coated paper, especially light weightcoated paper having only one cast surface, there has always existed aserious problem in respect to the tendency of the finished paper to curlrather than to lie fiat as desired. Many expedients have been suggestedfor removing or reducing the curl in such paper. In some cases suchpalliative treatments have been successful in reducing curl appreciably;in other cases they have failed to result in salable paper.

Curl in paper usually is due to strains or distortions introduced duringthe manufacture or processing of the paper Web. Whereas practice in thepast has been to relieve those strains insofar as possible or tocompensate for them, the present invention is concerned with seeking outthe cause of the strains and preventing their formation in the firstplace.

It has been found that when paper having a wet coating is passed underpressure between a metal roll and a flexible rubber-covered roll thepaper may be permanently stretched or elongated to a measurable extentinextreme cases as much as 5% or more. It has likewise been found that asthe amount of stretch or elongation of the paper increases, the greaterbecomes the tendency to curl and the more difficult it is to remove thecurl.

The major cause of stretch in roll-coated paper generally and incast-surface coated paper particularly has now been found to be thestretch that takes place in the surface of the rubber-coveredpressure-roll or backingroll when it passes through a pressure-nip. Whena web of paper is passed through a pressure-nip between a metal roll anda roll with a soft rubber surface, because of the high coefiicient offriction between the web and the rubber the paper tends to cling to therubber surface. Consequently if the rubber surface stretches the paperis likewise stretched or elongated to some extent thereby. Theconsiderable degree to which stretch occurs in the surface of a rubberroll has apparently hitherto gone unnoticed in connection with strainsand distortions in a paper web, and particularly by those who havemistakenly attributed curl in paper to strains caused by the changingradiusof the roll under pressure.

It is common to speak of a rubber-covered roll as being compressed in apressure-nip. It should be borne in mind, however, that solid elasticrubber, whether pure gum rubber or rubber reinforced by powder filler,is not to any appreciable degree truly compressible. That is to say,pressure causes only an exceedingly slight diminution in the volume ofthe rubber but instead causes a considerable displacement of the rubberin its position in space. In this respect rubber behaves like a liquid,which is substantially non-compressible but whose shape is dependentupon whatever forces act thereon. A layer of rubber forming the surfaceof a rubber-covered pressure-roll therefore is substantially unchangedin volume when pressed against another roll although its shape may bechanged considerably. It is apparent that if, as a rubber-covered rollis turning, the thickness or cross sectional area of the rubber layer isdecreased as it passes through the constriction of a pressure-nip, thenthe velocity of the layer must proportionately increase during passagethrough the constriction. If the thickness of the layer is decreased byone-half in the constriction, then obviously the corresponding velocityof the layer within the constriction must be doubled.

It is apparent that all portions of the elastic layer under constrictioncannot move with the same angular velocity. Obviously the inner side ofthe layer which is firmly attached to the solid metal core of the rollcannot vary in angular velocity. At a distance from the core, however,the extreme elasticity inherent in the rubber permits the layer to actsomewhat like a liquid so that the velocity in that part of the layer isconsiderably increased during passage through the nip constriction.Friction against the surface of the contacting drum very likely acts onthe outer surface of the rubber layer to prevent the velocity thereofreaching that of the interior of the layer. But nevertheless the changein velocity of the elastic surface is sufficient to stretch paper passedunder pressure between said elastic surface and a metal roll surface,and paper so stretched acquires a magnified tendency to curlobjectionably.

Having ascertained that a major cause of curling trouble in coated paperand particularly in drum-finished coated paper is the strain induced bythe elastic stretch of the surface of the rubber-covered backing-roll ina pressure nip, it became apparentthat the curl could be prevented,diminished, or controlled by control of the elastic stretch in thesurface bearing against the back of the paper in the pressure-nip. Thereare several ways for reducing elastic stretch in the backing surface.

Since the decrease in the cross-sectional area of the rubberlayer in thepressure nip is proportional to the depth of impression of the matingroll it is apparent that decreasing the applied pressure will decreasethe indentation in the rubber layer and correspondingly decrease itstendency to stretch. Likewise increasingthe hardness of the rubberlayers used will, for a given .applied pressure, decrease the depth ofindentation and so correspondingly decrease the tendency of the rubberto stretch. Both of these practices, however, are of limited use in mostcases, especially in the field of drum-finishing coated papers, becausein the newer high-speed processes for producing such paper bothconsiderable pressure and a considerable width of the pressure nip orbite are desirable.

Another method of decreasing the depth of indentation of the rubberlayer is to increase the diameter of the roll. Then for a given pressurethe width of nip is increased and the change in cross sectional area isdecreased. Increasing the thickness of the layer of rubber on thepressure-roll is helpful, since the percentage change in cross sectionalarea caused by any particular indentation is of course related to thethickness of the rubber layer. Thus a thick rubber layer, say of 5inches, is affected much less and causes less stretch and resultant curlin paper treated thereby than is the case with a thin rubber layer, sayone of only one inch in thickness.

However, the most generally satisfactory results in preventing orcontrolling curl inpaper passed through a pressure nip of appreciablewidth are achieved by using against the back of the paper web a surfacewhich itself does not stretch appreciably in the nip.

It has been found thatapaper web bearing on one side a layer of .wetaqueous coating composition may be pressed with its wet coated face intocontact with a metal roll through ,a. nip of appreciablewidthby means ofa yielding-surface roll which transmits pressure to the back of saidpaper web throughasurface which is .relativelyor substantially,non-stretching and that thereby substantially no stretch and littletendency to curl will be induced in the paper web. A satisfactory meansfor exerting pressure on the web to accomplish the desired results is aspecially designed roll having a rigid core, as of metal, over the corea layer of substantial thickness, preferably at least about f/z an inch,of soft, stretchable rubber or equivalent, and on the outside a thin,substantially non-stretching layer such as a layer of fabric or cordwhich is impregnated with or embedded in rubber which is harder and lessstretchable than the rubber of. the inner layer.

When such a roll passes through and is distorted in a pressure-nip, thestretch or velocity change is confined substantially wholly to the innerportion composed of soft stretchable rubber; the dimensions of thereinforced surface layer do not appreciably change. Consequently paperpassing through such a pressure nip is not appreciably stretched andlittle or no tendency towards curlingis induced by the action of thepressure nip.

It is realized that no roll can be prepared having a surface which isabsolutely non-stretchable. By substantially non-stretchable as usedherein is meant having a relatively very small degree of stretch incomparison with the stretch of elastic'rubber.

Instead 'of using 'a' pressure-roll of the construction described, asubstantially non-stretching endless belt may be inserted between thepaper-web and an ordinary soft rubber-covered pressure-roll as the paperpasses through a pressure-nip. Thisprocedure gives equally good resultsin regard to freedom from stretch and curl in the treated paperweb. Suchan endless belt, however, is somewhat more difiicult to prepare and keepin condition than is the previously described yielding roll with anonstretching surface. Less desirably the endless belt can be replacedby a non-stretching web of finite length.

A substantially non-stretching but yielding backing surface isadvantageous for use in any coating process where paper is passedthrough a pressure-nip. Its advantages are especially apparent inthefield of drum-finished or cast-coated paper because in that field itis customary to use backing rolls which are softer, and with resultantnip widths which are greater, than those generally used in other coatingprocesses.

Paper is made by filtration of a very dilute suspension in water ofcellulose fibers, in particular cellulose fibers derived from woodepulp,upon a moving foraminous surface or screen. The water escapes throughthe screen, leaving a layer of felted fibers on the surface of thescreen. The individual fibers are frornl to 5 millimeters in length,depending upon the particular tree from which they are obtained' Inspite of their shortlength, the fibers are relatively long and slenderin comparison with their breadth which isonly =a smallpercentage oftheir'length. In consequence the fibers as they settleout on the moving:sereentend to become oriented with their long dimension parallel to thedirection of travel of the screen. The paper web so formcdtherefore hasmany more ifibe'rs jlying substantially parallel to, its length than liesubstantially atiright anglestoiitslength. Paper is said to have a grainin the direction of travel of the screen upon which i-twas formedf'andits cross-grain direction is across or perpendicular to the direction oftravel of. the :screen.

Paper is very sensitive to the moisture in the surrounding. atmosphere,readily. absorbing moisture as the relative humidity rises iand'likewisegiving up moisture as .the relativehumidity falls. Paper changes indimensions as its moisturec'onte'nt varies. As water is absorbed, paperexpands; as water evaporates, paper contracts. Because ofthe'jorientation of :fibers in paper, the change in dimension greater inthe cross-grain direction than it is in the screen direction ordirection of the grain. The cross-grain change in dimension due tohumidity change lmaybe 2 01",3 times as great as in themachine-direction. When. layer. of coating composition is applied tonneside of such a web of paper, and the paper is thereafter subjected toatmospheres of varying relative humidity it is understandable that ifthe coating layer and the paper web change dimensions at different rateswith changes in the relative humidity then there may develop a tendencyfor the coated sheet to curl somewhat. Normally, coated paper isprocessed so that it will lie flat in an atmosphere having a relativehumidity of between 50% and 60%. Such one-side coated paper is generallyexpected to show no appreciable tendency to curl at relative humiditiesbe tween about 30% and about 80%. At greater extremes of relativehumidity such paper may be expected to show some tendency to curl, butthe curl, because of the grain of the paper, should be parallel to thegrain of the paper. That is, since the dimension change is greateracross the width of the web, the edges tend to curl toward the middle ofthe web to form a cylinder having its axis parallel to the direction ofscreen travel.

When a web of paper has been stretched and elongated as described above,it is found that it becomes more sensitive to humidity changes. Thus,for example, a coatedone-side paper which has not been stretched may liereasonably fl-at at all relative humidities between 30% and 70% but ifstretched moderately during its processing it may lie fiat only betweenrelative humidites of 40% and 60%. If the paper is elongated still more,the tendency to curl becomes more pronounced and surprisingly it may befound that the paper tends to curl toward the uncoated side at both highand low relative humidities. Moreover, if elongation is pronounced thepaper may tend to develop also a cross-grain curl, with its axis atright angles to the grain. In such a case the paper becomes almostunmanageable when it has been cut into sheets.

In one particular instance a paper web of the type customarily used asbodystock for coated paper, being made from a moderately beatencellulosic fibrous furnish containing a moderate amount of sizing agent,said web weighing about 40 pounds per 500 sheet ream 25 x 38 inches insize, was cast-coated on one side with about 18 pounds dry weight, perream of a typical clay and adhesive coating. Conditions at thepressure-nip on the casting drum were varied to produce various degreesof elongation in the paper web, the stretch varying from substantiallyzero percent to over 4 percent. In the region from substantially zeropercent to 0.25% stretch the sheets showed very little tendency to curlover a wide range of relative humidity, and there was no perceptibledifierence among the samples. In the range from 0.25% to 0.75% stretch,the curling tendency increased gradually but did not become especiallyobjectionable. Above 0.75 stretch the curl became objectionable. Atabout 1.5% stretch the paper became definitely unsalable because ofcrankiness caused by curling. At about 4% stretch the paper actuallyrolled up into a tight cylinder having its axis at 45 degrees to thegrain.

The preceding data are not given to establish definite limits, butmerely to show what happened with one specific example; It is known thatthe degree of curl at any degree of stretch will also depend, amongother conditions, upon the weight of the paper base, the degree ofhydration of the fibers therein, and the weight and composition of thecoating layer. If stretch can be avoided, however, the eifects of theother variables are greatly reduced and usually become insignificant.

The present invention controls the stretch or distortion in a paper webwhich is normally imparted to said paper web by passage through apressure nip having a surface of stretchable elastic substance, andthereby is effective in controlling curl in the paper web. In general itis desirable to control the stretch of the paper to less than 0.75%.Although theoretically it might be preferable to avoid all stretch inthe paper, actually controlling the stretch in the region of 0.25 isentirely satisfactory.

The invention will be further described and illustrated in connectionwith the accompanying drawings in which:

FIG. 1 is a diagrammatic side elevation of the. casting drum andpressure roller for the production of cast coated P p FIG. 2 is anenlarged cross section of the pressure roller of FIG. 1,

FIG. 3 is a diagrammatic side elevation of the casting drum and pressureroller with an endless belt surrounding the pressure roller,

. FIG. 4 is-an enlarged cross section of the pressure roller of FIG. 3,and

FIG. '5 is a cross section of the belt of FIG. 3.

Referring to FIGS. 1 and 2, 1 is the paper web coming from a source suchas a coating machine (not shown). The ,web passes on to the pressureroller 2, then downwardly through the nip between the pressure roller 2and the casting roller or drum 3 and around the casting drum 3 to thetake-01f roller 4. This is the conventional arrangement of the rollersin cast coating paper and the rollers are conventional excepting withrespect to the structure of the pressure roller 2. This roller, asstated above, has a flexible core or support for the outer surfacelayer. The specific embodiment illustrated consists of a rigid core 5 ofmetal, e.g. iron or steel, surrounded by a relatively thin layer 6 say Ainch thick of hard vulcanized rubber which in turn is surrounded by alayer 7, say /8 inch thick, of relatively soft elastic rubber having a Pand J plastometer value of about 50 and an outer layer 8 about A inchthick. The inner layer 6 of hard rubber serves only to ensure a goodbonding between the surface of the metal core 5 and the soft rubberlayer 7. The outer layer 8 is designed to be flexible but relativelynon-stretchable and is made of rubber having a P and J plastometer valueof about 35 with fabric or cords embedded therein, say linen cords of0.023 inch diameter laid side by side at a spacing of 12 per inch andextending around the roller in planes which are substantiallyperpendicular to the axis of the roller. The outer layer 8 is similar instructure to the well known cord or fabric reinforced rubber belts.

The P and J plastometer values referred to above are the distancesmeasured in 10 micron units which a /8 steel ball will sink into thesurface of a body of the rubber under a load of 1 kilogram.

The roller described above is about 15 inches in diameter. As has beenexplained the pressure roller may be of any suitable size withinreasonable limits, say 48 inches diameter and may have any desiredconstruction so long as it meets the basic requirements that it shallhave a substantially non-stretchable flexible surface layer supported byan elastic layer. The outer layer 8 should be sutficiently thin to bequite flexible and sufiiciently thick to be substantiallynon-stretchable and to permit a reasonable amount. of wear before thesurface layer of rubber wears off and exposes the embedded cord orfabric. The layer 7 may vary in thickness depending upon the size of theroller, its elasticity, the pressure to be applied to the rollers andthe desired width of the nip between the rollers. In this respect thepressure roller may be designed and constructed in accordance with theteachings of the prior art, the only novelty of our pressure. rollerbeing, as stated above, that it has a flexible, non-stretchable smoothsurface or surface layer. The layer 6 serves merely to provide a bondbetween the solid metal core of the roller and the elastic layer 7 andmay be omitted or substituted by other means of securing the elasticlayer 7 to the metal core. The casting drum or roller 3 is the usualwell known construction e.g. a metal drum having a polished chromiumsurface. The roller 4 is the conventional take-off roller. The paper web1 and the coating thereon may be the conventional paper and coatingsused in making cast coated paper. In fact the whole apparatusillustrated in FIG. 1 and the process carried out therein are well knownwith the single exception that the pressure roller has a flexiblenon-stretchable surface layer surrounding and covering the usual elasticlayer.

Referring to FIGS. 3, 4 and 5 the parts illustrated are conventionalwith the exception that the flexible, nonstretchable belt is interposedbetween the casting drum 3 and the pressure roller 2a. Here the pressureroller 2a also is conventional, consisting of a metal core 5, a bondinglayer 6 of hard rubber and an elastic layer 7 of soft rubber. In thisfigure We have shown a roller 11 for delivering the coated paper web 1to the nip between the casting drum 3 and the pressure roller 2a. Thetake-off roller 4 is the same as in FIG. 1. The two rollers 12 and 13serve to support the endless belt 10 which may be of the usualconstruction of rubber coated fabric or cord reinforced belts. Suchbelts generally are designed to have a smooth surface and to besubstantially non-stretchable. As shown in FIG. 5 the belt consists oflongitudinally extending parallel cords laid side by side, such as linencords 14 of 0.023 inch diameter, spaced 12 to the inch, embedded in awear resistant, flexible rubber matrix 15.

a The structure of the layer 8 in FIGS. 1 and 2 is similar to thestructure of the belt 10 as shown in FIG. 5.

As is apparent the belt '10 may be replaced by a strip of finite lengthwhich may be equal to the length of the paper web to be treated or maybe shorter and may be run through the nip repeatedly. Illustration ofthis obvi ous modification has been omitted as being unnecessary.

We wish to point out that any of the known methods of cast coating maybe followed. In this respect our method does not differ from the knownmethods. For instance the paper web may be coated and delivereddirectly, while the coating is still wet and plastic, to the nip betweenthe casting druinor the pressure roller. Or the coating may be appliedand dried and supercalendered if desired and then remoistened to aplastic state and then delivered into the nip between the casting drumand the pressure roller.

Any paper web suitable for cast coating may be used. As has been pointedout above our invention is especially useful in the manufacture of castcoated paper which has a tendency to curl objectionably i.e. lightweightpaper but is not limited to such paper. Conventional paper coatingcompositions, suitable for east coating and conventional weights thereofmay be used.

Our invention is not strictly limited to cast coating in any limitedsense or meaning of this term. Cast coating is sometimes understood tomean a coating process in which the coating on the paper webspontaneously adheres to the surface of the casting drum until itbecomes non-adhesive thereto due to solidification as a result of dryingout or cooling and then spontaneously releases from said surface. Ourinvention is applicable in any process in which a plastic coating on apaper web is pressed against a finishing surface and hardened in contactwith said surface and then separated from said surface.

Our invention not only serves to avoid or reduce the stretch imparted tothe paper web in the conventional cast coating process and thus toreduce the tendency of the paper to curl but also by avoiding stretchingthe paper web" avoids weakening the web and further by avoiding relativemovement between the casting surface and the surface of the coatinggives a more nearly perfeet cast surface. wrinkling and creping of thepaper web are avoided.

We claim:

1. Apparatus for simultaneously drying and drum finishing coated papercomprising: means for coating paper with an aqueous coating which isrelatively plastic when a predetermined quantity of moisture is presentin said coating; a finishing drum; means for heating said drum; meansfor applying said paper to said drum with said coated side in contactwith the surface of said drum and while said predetermined quantity ofmoisture is present in said coating; and means for pressingsaid paperagainst said drum and at least in part drying the same in contacttherewith by holding the same against said drum for a substantialinterval of time without simultaneously stretching the samesubstantially comprising a backing roll mounted for rotation in unisonwith and in pressure contact against said drum, an elastic resilientlayer on said backing roll, andthe surface of said resilient layer beingsubstantially non-stretchable While permitting said layer to yield undersaid pressure.

2. The apparatus defined in claim 1 further characterized by a textilereinforcement in the surface of said elastic resilient layer.

3. A process for simultaneously drying and drum finishing papercomprising the steps of: applying an aqueous coating containing a waterplasticizable adhesive to a paper base; applying said paper to a heated,convex, cylindrical finishing surface with the coated side toward thefinishing surface and with sufiicient water in said coating to rendersaid adhesive substantially plastic; pressing said coated paper againstsaid finishing surface for a substantial length of time and across asubstantial area by running it through a hip created by an elasticresilient backing roll pressing against said finishing drum; andsimultaneously with said pressing step preventing said paper fromstretching by applying a pressure to hold the paper against thecylindrical surface over a substantial area and advancing the backsurface of the paper with the applied pressure at a uniform speed andtension through said area.

4. The process of simultaneously dryingand drum finishing paper definedin claim 3 further characterized by bringing said coating intorelatively moist adhesive contact with said finishing drum, and holdingthe same in said contact until said adhesive is substantially dry.

References Cited in the tile of this patent UNITED STATES PATENTS2,312,853 Toland et al. Mar. 2, 1943 2,316,202 Warner Apr. 13, 19432,324,631 May July 20, 1943 2,442,443 Swallow June 1, 1948 2,597,858Freelander May 27, 1952 FOREIGN PATENTS 527,207 Canada July 3, 1956

1. APPARATUS FOR SIMULTANEOUSLY DRYING AND DRUM FINISHING COATED PAPERCOMPRISING: MEANS FOR COATING PAPER WITH AN AQUEOUS COATING WHICH ISRELATIVELY PLASTIC WHEN A PREDETERMINED QUANTITY OF MOISTURE IS PRESENTIN SAID COATING, A FINISHING DRUM, MEANS FOR HEATING SAID DRUM, MEANSFOR APPLYING SAID PAPER TO SAID DRUM WITH SAID COATED SIDE IN CONTACTWITH THE SURFACE OF SAID DRUM AND WHILE SAID PREDETERMINED QUANTITY OFMOISTURE IS PRESENT IN SAID COATING, AND MEANS FOR PRESSING SAID PAPERAGAINST SAID DRUM AND AT LEAST IN PART DRYING THE SAME IN CONTACTTHEREWITH BY HOLDING THE SAME AGAINST SAID DRUM FOR A SUBSTANTIALINTERVAL OF TIME WITHOUT SIMULTANEOUSLY STRETCHING THE SAMESUBSTANTIALLY COMPRISING A BACKING ROLL MOUNTED FOR ROTATION IN UNISONWITH AND IN PRESSURE CONTACT AGAINST SAID DRUM, AN ELASTIC RESILIENTLAYER ON SAID BACKING ROLL, AND THE SURFACE OF SAID RESILIENT LAYERBEING SUBSTANTIALLY NON-STRETCHABLE WHILE PERMITTING SAID LAYER TO YIELDUNDER SAID PRESSURE.